@misc{cryptoeprint:2011:626,
author = {Nicolas T. Courtois},
title = {Algebraic Complexity Reduction and Cryptanalysis of GOST},
howpublished = {Cryptology ePrint Archive, Report 2011/626},
year = {2011},
note = {\url{http://eprint.iacr.org/}},
}
You will need the url.sty package (comes with most LaTeX installations) for the last line. Otherwise, remove the \url command.